1. Field Of the Invention
This invention pertains to a method and apparatus for three dimensional modeling and, more particularly, to a method for controlling the exposure of photoformable layers to produce models exhibiting reduced distortion due to internal stresses.
2. Description of Related Art
Many systems for production of three-dimensional (solid) modeling by photoforming are in existence. European Patent Application No. 250,121 filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents pertinent to this art. U.S. Pat. No. 4,575,330 issued Mar. 11, 1986, to C. W. Hull, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminas adhering to each other, representing corresponding successive adjacent cross-sections of the object, are formed resulting in a three-dimensional object. U.S. Pat. No. 4,752,498 issued Jun. 21, 1988, to E. V. Fudim, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material in contact with the uncured photopolymer. The transmitting material is one that leaves the irradiated surface capable of further cross-linking, so that a subsequently formed layer will adhere thereto.
In generating a three-dimensional object using successive layer hardening by exposure to an imagewise modulated hardening radiation to form successive cross-sections of the object, it is important to assure that each layer accurately represents the desired cross-section of the object, so as to generate a three-dimensional object which is an accurate representation of the desired object. Therefore, it is important that there be no distortion introduced in the creation of each layer which will result in a distorted object. Unfortunately, distortions in the individual layers do occur as the photoformable composition changes during the irradiation process from a substantially freely flowing state to a substantially hardened solid state, due to stresses in the layers. Such stresses are believed to be the result of substantial forces generated during molecular shrinkage as a result of the hardening irradiation process. Numerous solutions to this problem have been proposed.
One solution involves exposing the layers to a WEAVE pattern, discussed in Chapter 8, Advanced Part Building, pages 195-219, "RAPID PHOTOTYPING & MANUFACTURING, Fundamentals Of Stereo Lithography" First Edition, by Paul F. Jacobs, Published by The Society of Manufacturing Engineers, One SME drive, P.O. Box 930, Dearborn, Mich. 48121-0930. U.S. Pat. No. 4,945,032 issued Jul. 31, 1990, to Murphy et al., proposes a process for the reduction of distortions whereby the exposure of a layer is stopped and then repeated at least once. Japanese Patent application 63[1988]-172685, published Jan. 26, 1990, also uses two exposures, a first imagewise exposure which leaves only partially cured resin in the exposed areas and, after the model is completed, a second overall exposure which hardens the partially cured resin to produce a solid model. While these methods may reduce distortion, there is still need for a method that will result in an undistorted solid model, having minimal uncured material trapped in its body and maximum hardness.